Field of the Invention
The invention relates to a method for decomposing nitrogen oxides in an exhaust gas of an internal combustion engine, in which the exhaust gas and a reactant that is sprayed into the exhaust gas through the use of compressed air are fed to a catalytic converter. Moreover, the invention relates to a device for decomposing nitrogen oxides in the exhaust gas of an internal combustion engine, having a catalytic converter for receiving the exhaust gas and a reducing agent which is to be sprayed into the exhaust gas through the use of compressed air, and a spraying device for spraying the reducing agent.
The principle of a regulated or controlled diesel catalytic converter (CDC) has proved to be expedient for reducing pollutants, especially nitrogen oxides, contained in the exhaust gas of an internal combustion engine. That principle is applied primarily in internal combustion engines which are operated with excess air, such as, for example, in diesel and lean-burn engines. The principle, which is based essentially on the method of selective catalytic reduction (SCR), has in the meantime been disclosed in numerous publications, e.g. in German Published, Non-Prosecuted Patent Application DE 43 10 926 A1, German Published, Non-Prosecuted Patent Application DE 43 15 278 A1 corresponding to U.S. Pat. No. 5,628,186, and European Patent 0 617 199 B1 corresponding to co-pending U.S. application Ser. No. 08/490,115, filed Jun. 12, 1995. In the SCR method, the nitrogen oxides are contacted, together with ammonia, on a selective catalyst and are converted there into nitrogen and water.
Due to the problems associated with the use of ammonia, namely the toxicity and the potential noxious odor, ammonia as such should not be carried in the vehicle if the internal combustion engine is fitted with a CDC system. The reactant which is necessary for the catalytic conversion of the nitrogen oxides is therefore transported in the vehicle in the form of an aqueous solution of urea or the like. The ammonia is produced from the aqueous solution by hydrolysis, in each case in the quantity required to convert the nitrogen oxides which have been generated. Pure ammonia or ammonia solution can be used in a stationary flue gas purification system, e.g. in the exhaust gas line of a power plant.
In accordance with German Published, Non-Prosecuted Patent Application DE 44 17 238 A1, provision is made for the exhaust-gas line of a diesel engine of a truck to be fed directly up to the side of a cylindrical inlet chamber in which a perforated plate shaped to form a funnel is disposed. An aqueous urea solution is injected into the inner space of the funnel through the use of an injection valve provided at the narrowest point of the funnel. In that way, a homogeneous distribution of the urea solution in the exhaust gas is achieved over the entire cross section of the inlet chamber. Adjoining the inlet chamber are a hydrolysis catalyst, a DeNO.sub.x catalyst and, if appropriate, an oxidation catalyst.
A solution which is described in European Patent Application 0 586 913 A2 differs from the above. In order to obtain sufficient nebulization of the medium to be nebulized, i.e. an aqueous solution of urea, a mixing device or premixing chamber is provided upstream of where it is introduced into the flow of exhaust gas containing pollutants. The medium and a compressed gas, such as air, are introduced into the premixing chamber for intimate mixing with one another, i.e. to form a dispersion. The premixing chamber is connected through the use of a mixing line or pipe to a nebulizing nozzle which, in turn, is disposed in the flow of exhaust gas. It is possible to achieve quite a good distribution of the aqueous solution in the flow of exhaust gas with the premixing and the spraying-in. In that case, the use of the compressed air also brings about cooling of the nebulizing nozzle located in the hot flow of exhaust gas.
The compressed air which is required for the injection and spraying-in is usually produced in a special compressor that has to be provided specially, which incurs costs.